Higher alcohol synthesis from syngas over xerogel-derived Co-Cu-Al2O3 catalyst with an enhanced metal proximity

Abstract

To improve metal proximity of Co-Cu-Al2O3 catalyst, xerogel-derived mesoporous Co-Cu-Al2O3 catalysts (CoCuAl-Xe) with various Co:Cu molar ratios (3:1, 2:1, 1:1, 1:2, and 1:3) were prepared by an epoxide-driven sol-gel method and applied to higher alcohol synthesis from syngas. The CoCuAl-Xe catalyst induced the formation of a composite spinel structure containing Co, Cu, and Al. At the optimal Co/Cu ratio, the Cu atoms efficiently substituted the Co2+ ions in Co3O4 to form the CuCo2O4 spinel structure. However, an excess amount of Cu induced segregation of CuO, leading to decreasing Co-Cu proximity. In higher alcohol synthesis from syngas, all the catalysts showed the linear quasi-Anderson Schulz Flory plot of carbon-containing products, which was attributed to the CO insertion mechanism in higher alcohol synthesis. Among the xerogel catalysts, 2Co1CuAl-Xe exhibited the highest CO conversion and higher alcohol yield. The Co-rich CoCuAl-Xe catalyst was disadvantageous in terms of alcohol selectivity because of a lack of CO insertion sites, and the Cu-rich one was disadvantageous in CO dissociation and led to methanol formation. The computational study suggests that high Co-Cu proximity favors CHOH* mechanism for CH* formation and promotes CO insertion reaction. Consequently, it was found that the optimal Co/Cu ratio and high Co-Cu proximity is required for high catalytic activity in higher alcohol synthesis.